This invention relates to carbon materials. The invention particularly relates to thermally anisotropic carbon materials.
Among their many applications, carbon materials are used for heat management purposes. This is particularly critical in electronics applications. More compact and sophisticated electronic devices and advances in semiconductor technology have resulted in rising transistor density and switching speed of microprocessors. This has been accompanied by drastic increases in heat production.
In response to this challenge, commercial companies are focusing on developing high performance, low cost, compact, and reliable schemes to handle these very large thermal loads.
The technologies proposed to date rely on the concepts of heat spreaders and heat sinks and various combinations of spreaders and sinks.
A heat spreader is an article that spreads heat quickly. A heat spreader requires high thermal conductivity and low heat capacity.
A heat sink is an article that absorbs heat quickly. It stores heat so it needs high thermal conductivity and high heat capacity.
Conventionally in electronics, heat is spread out by a heat spreader to a heat sink and then removed from the heat sink to the surroundings either by natural convection or by forced flow of a coolant (e.g. fan cooling). For more demanding applications other solutions such as heat pipes or liquid cooled systems may be required.
A carbon based material that has been proposed for use as a heat spreader is diamond. Diamond has the highest known thermal conductivity of any material. However it is currently expensive to make. Diamond is also isotropic in nature, the thermal conductivity in any one direction being the same as in any other direction. This means that as well as spreading heat, a diamond heat spreader allows heat to pass through it to the side remote the source of heat. This can be disadvantageous in applications where components are in close proximity.
A thermally anisotropic carbon material is graphite. The crystal structure of graphite comprises layers within which there is strong bonding, with weak bonding between the layers. Additionally, within each layer there are delocalised electrons. This structure leads to a high degree of anisotropy, with thermal (and electrical) conductivity within the plane being very much higher than thermal (and electrical) conductivity through the plane.
To exploit this anisotropy, several proposals have been made for graphite based heat spreaders and sinks, for example among recent proposals are:—                U.S. Pat. No. 6,746,768 which discloses a thermal interface material comprising a flexible graphite sheet containing oil which reduces thermal resistance when attached to a component.        U.S. Pat. No. 6,771,502 which discloses a finned heat sink constructed from the above resin-impregnated graphite sheets.        U.S. Pat. No. 6,758,263 which discloses an anisotropic laminated graphite heat sink made of the above resin-impregnated graphite sheets which has a cavity into which is inserted a thermally conductive material. The heat from a heat source can be conducted via the core and into the thickness of the heat sink and then out across the plane of the heat sink.        U.S. Pat. No. 6,841,250 which discloses heat sink designs using such laminated graphite sheets for conducting heat away from an electronic component and dissipating it through the heat sink.        U.S. Pat. No. 6,777,086 which discloses resin impregnated exfoliated graphite sheets which are resin-impregnated (5-35% wt) and calendered to 0.35-0.50 mm.        U.S. Pat. No. 6,503,626 which discloses that such resin impregnated graphite sheets may be comminuted, pressed & cured to form a block which can then be machined into a desired finned shape.        U.S. Pat. No. 6,844,054 which discloses resin impregnated carbon fibre heat sinks of various geometric designs (cones, pyramids, domes, etc.).        U.S. Pat. No. 6,119,573 which discloses the use of carbon fibre material as a thermally conductive interface between a missile housing and an electronics package to give a low weight high thermal conductivity heat sink.        U.S. Pat. No. 5,837,081 which discloses a composite produced from a mat of graphitised (to 2800° C.) vapour grown fibre (i.e. highly oriented pyrolytic graphite—HOPG) which is densified by chemical vapour deposition (CVD) of pyrolytic carbon.        U.S. Pat. No. 6,514,616 which proposes the use of highly oriented pyrolytic graphite encapsulated in polyimide, epoxy or other polymer.        US 2006/0029805 [published after the earliest priority date of the present application] discloses the idea of hot pressing graphites with a mesophase pitch or phenolic resin as a binder and heat treating to graphitise the binder. US 2006/0029805 discloses that pressing preferably aligns the graphite perpendicular to the moulding direction and that such composite materials have high thermal conductivities (paragraph [0040] shows thermal conductivities of 204.4 W/mK and 76.8 W/mK in the in-plane and through-plane directions for a mesophase pitch binder material). US 2006/0029805 also discloses that mesophase pitch binder materials have a higher thermal conductivity than resin binder materials.        
Other patents using graphite's anisotropic thermal conductivity for thermal management include U.S. Pat. No. 4,878,152, U.S. Pat. No. 5,542,471, U.S. Pat. No. 6,208,513, U.S. Pat. No. 5,523,260, U.S. Pat. No. 5,766,765, U.S. Pat. No. 6,027,807, U.S. Pat. No. 6,131,651.
Despite this widespread use of graphite the performance and cost of graphite based materials varies widely. This is because the degree of anisotropy depends upon the degree of orientation of the graphite. To obtain a highly oriented graphite is difficult.
In their patent application WO02/090291 the applicants proposed a method of forming a graphite material comprising the steps of:—
a) forming under high shear a mouldable composition comprising:—
                i) graphite powder; and        ii) a binder; and        iii) a fluid carrierb) working said mouldable composition under high shear to form an extruded shapec) forming bodies from said shape;d) heat treating said bodies to stabilise the structure; ande) machining the bodies to form features in their surfaces.        
The high shear working of the compositions could be by rolling.
WO02/090291 concentrated on aqueous binders but did mention the possibility of using pitch based binders. The purpose behind WO02/090291 was to provide a highly graphitic body without the need for a high temperature graphitisation step
Pitch comes in many forms. One such form is mesophase pitch (sometimes called liquid crystal pitch). Mesophase pitch is a partially pyrolysed material containing highly linked aromatic groups and is in effect a pitch part way through conversion to graphite. Continued pyrolysis results in graphitisation. Mesophase pitch is sometimes used as a matrix material in carbon-carbon composites to bind carbon fibres.